177 



the latter species, in which the skeleton is reduced to a pair of simple, 

 small rods, there is only a very small step to the one of Krohn's larvae, 

 (0. elongatus) which was observed by that author also to have a rudiment- 

 ary skeleton ( unfortunately, he does not give any figures of it ). We 

 have thus an unbroken series from the typical Ophiopluteus through the 

 worm-shaped Ophiopluteus elonyalus with a larval skeleton to the larvae of 

 Ophioderma brevispina and Ophionereis squamulosa with no larval skeleton, 

 and further on to Ophionereis Shai/eri with the larval stage completely 

 eliminated. 



Amphiura vivipara H. L. Clark. 



PI. XXXI, Figs. 7 9. 



The discovery of this new viviparous Amphiura was one of the results 

 of the Carnegie Expedition to Tobago, B.W. I., in April lOlf) 1 ). As empha- 

 sized by Ludwig in his paper on "Brutpflege bei Echinodermen" 2 ) the 

 viviparous Ophiuricls hitherto known all belong to the colder regions (arc- 

 tic-subarctic or antarctic-subantarctic), with the sole exception, besides 

 the nearly cosmopolitan Amphiura (or, more correctly, Amphipholis) squa- 

 mata, of Hemipholis cordifera Lyman (elongata Say); moreover, the vivi- 

 parity of the latter is, no doubt, only apparent, as I have shown recently. 3 ) 

 It was then of considerable interest to find a truly viviparous Ophiurid 

 living here, even under the most tropical conditions, viz. on a coral reef. 

 (It was mainly found in thick cushions of Corallina, which covered large 

 patches on the reef lying dry at ebb tide). 



It was at once evident from the considerable size of the eggs, 0.5 mm, 

 that the interest attached to this case was more than that of having found 

 a tropical viviparous Ophiurid. In the only viviparous Ophiurid hitherto 

 studied, Amphipholis squamala, the eggs are very small, only ca. 0.15 mm, 

 and the cleavage total and regular; the embryo has a rudimentary larval 

 skeleton and is, evidently enough, only a reduced Pluteus. In the present 

 species all this is different. First of all the cleavage is meroblastic. 

 The nuclei, in the first cleavage stages, lie irregularly spread in the yolk 

 substance and there is no trace of cell division ; then they gradually arrange 

 themselves in a more regular order along the surface of the egg, forming 

 thus the ectoderm (PI. XXXI, Figs. 78), while in the interior they remain 

 without distinct order till a much later stage. The details of the embryonal 

 and postembryonal development cannot be given here, as I cannot spare 



1 ) H. L. Clark. Brittle-Stars, new and old. Bull. Mus. Comp. Zool. LXII. 6. 1918. p. 268. 



2 ) Zoologische Jahrbucher. Suppl. VII. 1904. 



3 ) Th. Mortensen. On Herinaphroditism in viviparous Ophiurids. Acta Zoologica. I. 

 1020. p. 45. 



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